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Internet Engineering Task Force (IETF)                        J. ReschkeRequest for Comments: 7694                                    greenbytesCategory: Standards Track                                  November 2015ISSN: 2070-1721Hypertext Transfer Protocol (HTTP) Client-Initiated Content-EncodingAbstract   In HTTP, content codings allow for payload encodings such as for   compression or integrity checks.  In particular, the "gzip" content   coding is widely used for payload data sent in response messages.   Content codings can be used in request messages as well; however,   discoverability is not on par with response messages.  This document   extends the HTTP "Accept-Encoding" header field for use in responses,   to indicate the content codings that are supported in requests.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7694.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Reschke                      Standards Track                    [Page 1]

RFC 7694                        HTTP CICE                  November 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Notational Conventions  . . . . . . . . . . . . . . . . . . .23.  Using the 'Accept-Encoding' Header Field in Responses . . . .34.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .45.  Deployment Considerations . . . . . . . . . . . . . . . . . .46.  Security Considerations . . . . . . . . . . . . . . . . . . .57.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .57.1.  Header Field Registry . . . . . . . . . . . . . . . . . .57.2.  Status Code Registry  . . . . . . . . . . . . . . . . . .68.  References  . . . . . . . . . . . . . . . . . . . . . . . . .68.1.  Normative References  . . . . . . . . . . . . . . . . . .68.2.  Informative References  . . . . . . . . . . . . . . . . .6   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .7   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .71.  Introduction   In HTTP, content codings allow for payload encodings such as for   compression or integrity checks ([RFC7231], Section 3.1.2).  In   particular, the "gzip" content coding ([RFC7230], Section 4.2) is   widely used for payload data sent in response messages.   Content codings can be used in request messages as well; however,   discoverability is not on par with response messages.  This document   extends the HTTP "Accept-Encoding" header field ([RFC7231],   Section 5.3.4) for use in responses, to indicate the content codings   that are supported in requests.  It furthermore updates the   definition of status code 415 (Unsupported Media Type) ([RFC7231],   Section 6.5.13), recommending that the "Accept-Encoding" header field   be included when appropriate.2.  Notational Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   This document reuses terminology defined in the base HTTP   specifications, namelySection 2 of [RFC7230] andSection 3.1.2 of   [RFC7231].Reschke                      Standards Track                    [Page 2]

RFC 7694                        HTTP CICE                  November 20153.  Using the 'Accept-Encoding' Header Field in ResponsesSection 5.3.4 of [RFC7231] defines "Accept-Encoding" as a request   header field only.   This specification expands that definition to allow "Accept-Encoding"   as a response header field as well.  When present in a response, it   indicates what content codings the resource was willing to accept in   the associated request.  A field value that only contains "identity"   implies that no content codings were supported.   Note that this information is specific to the associated request; the   set of supported encodings might be different for other resources on   the same server and could change over time or depend on other aspects   of the request (such as the request method).Section 6.5.13 of [RFC7231] defines status code 415 (Unsupported   Media Type) to apply to problems related to both media types and   content codings.   Servers that fail a request due to an unsupported content coding   ought to respond with a 415 status and ought to include an "Accept-   Encoding" header field in that response, allowing clients to   distinguish between issues related to content codings and media   types.  In order to avoid confusion with issues related to media   types, servers that fail a request with a 415 status for reasons   unrelated to content codings MUST NOT include the "Accept-Encoding"   header field.   It is expected that the most common use of "Accept-Encoding" in   responses will have the 415 (Unsupported Media Type) status code, in   response to optimistic use of a content coding by clients.  However,   the header field can also be used to indicate to clients that content   codings are supported, to optimize future interactions.  For example,   a resource might include it in a 2xx response when the request   payload was big enough to justify use of a compression coding but the   client failed do so.Reschke                      Standards Track                    [Page 3]

RFC 7694                        HTTP CICE                  November 20154.  Example   A client submits a POST request using the "compress" content coding   ([RFC7231], Section 3.1.2.1):     POST /edit/ HTTP/1.1     Host: example.org     Content-Type: application/atom+xml;type=entry     Content-Encoding: compress     ...compressed payload...   The server rejects the request because it only allows the "gzip"   content coding:     HTTP/1.1 415 Unsupported Media Type     Date: Fri, 09 May 2014 11:43:53 GMT     Accept-Encoding: gzip     Content-Length: 68     Content-Type: text/plain     This resource only supports the "gzip" content coding in requests.   At this point, the client can retry the request with the supported   "gzip" content coding.   Alternatively, a server that does not support any content codings in   requests could answer with:     HTTP/1.1 415 Unsupported Media Type     Date: Fri, 09 May 2014 11:43:53 GMT     Accept-Encoding: identity     Content-Length: 61     Content-Type: text/plain     This resource does not support content codings in requests.5.  Deployment Considerations   Servers that do not support content codings in requests already are   required to fail a request that uses a content coding.Section 6.5.13 of [RFC7231] defines the status code 415 (Unsupported   Media Type) for this purpose, so the only change needed is to include   the "Accept-Encoding" header field with the value "identity" in that   response.Reschke                      Standards Track                    [Page 4]

RFC 7694                        HTTP CICE                  November 2015   Servers that do support some content codings are required to fail   requests with unsupported content codings as well.  To be compliant   with this specification, servers will need to use the status code 415   (Unsupported Media Type) to signal the problem and will have to   include an "Accept-Encoding" header field that enumerates the content   codings that are supported.  As the set of supported content codings   is usually static and small, adding the header field ought to be   trivial.6.  Security Considerations   This specification only adds discovery of supported content codings   and diagnostics for requests failing due to unsupported content   codings.  As such, it doesn't introduce any new security   considerations over those already present in HTTP/1.1 (Section 9 of   [RFC7231]) and HTTP/2 (Section 10 of [RFC7540]).   However, the point of better discoverability and diagnostics is to   make it easier to use content codings in requests.  This might lead   to increased usage of compression codings such as gzip (Section 4.2   of [RFC7230]), which, when used over a secure channel, can enable   side-channel attacks such as BREACH (seeSection 10.6 of [RFC7540]   and [BREACH]).  At the time of publication, it was unclear how   BREACH-like attacks can be applied to compression in HTTP requests.7.  IANA Considerations7.1.  Header Field Registry   HTTP header fields are registered within the "Message Headers"   registry located at <http://www.iana.org/assignments/message-headers>, as defined by [BCP90].   This document updates the definition of the "Accept-Encoding" header   field.  The "Permanent Message Header Field Names" registry has been   updated as follows:   +-----------------+----------+----------+---------------------------+   | Header Field    | Protocol | Status   | Reference                 |   | Name            |          |          |                           |   +-----------------+----------+----------+---------------------------+   | Accept-Encoding | http     | standard |Section 5.3.4 of          |   |                 |          |          | [RFC7231] andSection 3   |   |                 |          |          | of this document          |   +-----------------+----------+----------+---------------------------+Reschke                      Standards Track                    [Page 5]

RFC 7694                        HTTP CICE                  November 20157.2.  Status Code Registry   HTTP status codes are registered within the "HTTP Status Codes"   registry located at <http://www.iana.org/assignments/http-status-codes>.   This document updates the definition of the status code 415   (Unsupported Media Type).  The "HTTP Status Codes" registry has been   updated as follows:   +-------+-----------------+-----------------------------------------+   | Value | Description     | Reference                               |   +-------+-----------------+-----------------------------------------+   | 415   | Unsupported     |Section 6.5.13 of [RFC7231] and Section |   |       | Media Type      | 3 of this document                      |   +-------+-----------------+-----------------------------------------+8.  References8.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Message Syntax and Routing",RFC 7230, DOI 10.17487/RFC7230, June 2014,              <http://www.rfc-editor.org/info/rfc7230>.   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Semantics and Content",RFC 7231,              DOI 10.17487/RFC7231, June 2014,              <http://www.rfc-editor.org/info/rfc7231>.8.2.  Informative References   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration              Procedures for Message Header Fields",BCP 90,RFC 3864,              September 2004, <http://www.rfc-editor.org/info/bcp90>.   [BREACH]   Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the              CRIME Attack", July 2013,              <http://breachattack.com/resources/              BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.Reschke                      Standards Track                    [Page 6]

RFC 7694                        HTTP CICE                  November 2015   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext              Transfer Protocol Version 2 (HTTP/2)",RFC 7540,              DOI 10.17487/RFC7540, May 2015,              <http://www.rfc-editor.org/info/rfc7540>.Acknowledgements   Thanks go to the Hypertext Transfer Protocol Working Group   participants, namely Amos Jeffries, Ben Campbell, Mark Nottingham,   Pete Resnick, Stephen Farrell, and Ted Hardie.Author's Address   Julian F. Reschke   greenbytes GmbH   Hafenweg 16   Muenster, NW  48155   Germany   Email: julian.reschke@greenbytes.de   URI:http://greenbytes.de/tech/webdav/Reschke                      Standards Track                    [Page 7]